The invention relates to internal combustion engines and more particularly to the control of the combustible mixture for internal combustion engines. Still more particularly the invention relates to an apparatus for controlling the flow of gas to and from the induction manifold of the engine in order to influence the composition of the combustible mixture and hence also influence the composition of the exhaust gas. In a known apparatus of this type, for example that made known by the German Offenlegungsschrift No. 24 31 046, there is provided a bypass channel around the main throttle valve of the engine for admitting additional fresh air to the combustible mixture. The bypass channel is controlled by a throttle which is actuated pneumatically. The control pressure for the pneumatic control element is taken from the narrowest portion of the venturi of the induction tube and may be modified by a solenoid valve according to the prevailing ambient pressure. The valve is actuated in dependence on the output signal from an oxygen sensor located in the exhaust system which provides pulses of constant frequency but variable length. The known controller for this valve includes several differential amplifiers and a sawtooth generator and thus is relatively expensive and is sensitive and subject to malfunctions.